Open vowel

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An open vowel is a vowel sound in which the tongue is positioned as far as possible from the roof of the mouth. Open vowels are sometimes also called low vowels in reference to the low position of the tongue. The open vowels with dedicated symbols in the International Phonetic Alphabet are:

• open front unrounded vowel [a]
• open front rounded vowel [ɶ]^[1]
• open back unrounded vowel [ɑ]
• open back rounded vowel [ɒ]

There also are mid vowels which do not have dedicated symbols in the IPA:

• open near-front unrounded vowel [æ̞̈] (commonly written [a])
• open near-front rounded vowel [ɶ̈] or [ɶ̠]
• open central unrounded vowel [ɐ̞] (commonly written [ä], but that could also denote a near-front vowel)
• open near-back unrounded vowel [ɑ̈] or [ɑ̟]
• open near-back rounded vowel [ɒ̈] or [ɒ̟]

There is not an unambiguous way of transcribing an open central rounded vowel, but one could use one of the following symbols: [ɶ̈], [ɶ̠], [ɒ̈] or [ɒ̟]. The first two can denote either a near-front or central vowel, while the last two either a near-back or central vowel.
In the context of the phonology of any particular language, a low vowel can be any vowel that is more open than a mid vowel. That is, open-mid vowels, near-open vowels, and open vowels can all be considered low vowels.